Separation and recovery of liquid fractions



March 24, 1970 R. E. BRIDGEFORD 3,502,547

SEPARATION AND RECOVERY OF LIQUID FRACTIONS Filed April 27, 1966 PRIMARYOVERHEAD (PRO PANE) FEED (ALKYLATION EFFLUENT) VAPOR DOWNCOMER '-lSECONDARY OVERHEAD IS OBUTA N E) BOTTOMS (ALKYLATE) INVENTOR R. E.BRIDGEFORD "if A TTORNEYS United States Patent 3,502,547 SEPARATION ANDRECOVERY OF LIQUID FRACTIONS Ronald E. Bridgeford, Bartlesville, Okla,assignor to Phillips Petroleum Company, a corporation of Delaware FiledApr. 27, 1966, Ser. No. 545,664 Int. Cl. B0111 3/22 U.S. Cl. 203-71 8Claims ABSTRACT OF THE DISCLOSURE A feed stream comprising propane,isobutane and C alkylate is introduced into the top section of a singlefractional distillation column having a top section and a bottom sectionseparated by a solid, vapor impermeable plate. At least one downcomer,which serves as the only fluid passageway through said plate, extendsdownwardly into the liquid on a tray in the top portion of the bottomsection to permit the passage of only liquid from the top section to thebottom section while preventing the passage of vapor from the bottomsection to the top section. Each section is provided with means forreboiling the liquid contained therein. An overhead product streamcontaining propane is withdrawn from the top of the top section while anintermediate stream containing isobutane is withdrawn from the top ofthe bottom section. The bottom section can have a smaller diameter thanthe top section.

This invention relates to the separation and recovery of liquidfractions. In one of its aspects, it relates to an apparatus forrecovery of a middle boiling hydrocarbon fraction comprising afractional distillation tower containing a top section and -a bottomsection, means for passing a mixture of hydrocarbons to the top portionof the top section of the column, means for removing gaseous overheadfrom the top section of the column, a solid plate separating the topsection from the bottom section, a means to allow liquid only to passfrom the top section to the bottom section, reboiler means in the lowerportion of the top section, reboiler means in the lower portion of thebottom section, means for removing vapor from the upper portion of thebottom section, means for removing liquid from the lower portion of thebototm section. In another of its aspects, the invention relates to anapparatus for recovering a middle boiling hydrocarbon fraction ashereinbefore described wherein said means to allow liquid only to passfrom the top section to the bottom section comprises a downcomer pipethrough said solid plate in combination with a doughnut tray in the topportion of the bottom section. In a still further aspect, the inventionrelates to an apparatus for recovery of a middle boiling hydrocarbonfraction as hereinbefore described wherein the bottom section of thecolumn is smaller in diameter than the top section.

The invention also relates to a process for recovering a middle boilingfraction from a feed stream comprising passing said stream to the upperportion of a fractional distillation zone containing an upper zone and alower zone, the upper zone being separated from the lower zone by animpermeable plate adapted to pass liquid only from the upper zone to thelower zone, reboiling liquid in the upper zone, passing liquid at bubblepoint from the upper zone to the lower zone, reboiling liquid in thebottom zone, removing vapor from the upper portion of the bottom zoneand removing liquid from the lower portion of the bottom zone.

In the alkylation of, for example, ethylene with isobutane, the eflluentfrom the reaction contains propane, isobutane and an alkylate, theisobutane boiling at a 3,502,547 Patented Mar. 24, 1970 temperaturebetween that of the alkylate and the propane. It is desirable toseparate the products so that the alkylate can be recovered and so thatthe isobutane can be recycled.

The three components of the efiluent can be separated in a two stripperdevice wherein the feed is passed to the first stripper and the lowerboiling product, the propane, is removed overhead, and the isobutane andalkylate are removed as liquid and passed to the second stripper whereinthe isobutane is removed overhead from the second stripper.

A less expensive method and apparatus for separating the effluentproducts is a single column stripper in which isobutane is removed as avaporous side draw in a central portion of the column, propane isremoved overhead and alkylate is removed as a liquid from the bottomportion of the column. Most prior art single strippers are rather largein that they require a reboiler in the bottom portion of the column toheat the entire column. Some prior art apparatus provide a reboilingmeans in a middle portion of the column.

I have now discovered that the apparatus for separating these componentscan be substantially reduced in cost and that the cost of separating thecomponents with steam supplied to the column can be substantiallyreduced if a vapor impermeable plate is provided in a singlefractionation column between an upper section and a lower section,liquid in the upper section is reboiled above the plate, liquid in thelower section is reboiled, liquid in the upper section is passed to thelower section, and a process stream of intermediate volatility isremoved as a gaseous stream from an area below the vapor impermeableplate.

By various aspects of this invention, one or more of the following orother objects can be obtained.

It is an object of this invention to provide a process and apparatus forseparating a hydrocarbon stream from a mixture of hydrocarbons whereinthe desired stream boils intermediate the other components of the feed.

It is a further object of this invention to provide an improvedapparatus for separating a plurality of hydrocarbon components whereinthe size and cost of the equip ment is substantially reduced.

It is -a still further object of this invention to provide an improveddistillation process and apparatus for the separation of hydrocarbonmixture wherein the heating costs are substantially reduced.

Other aspects, objects, and the several advantages of the invention areapparent to one skilled in the art from a study of this disclosure, thedrawing and the appended claims.

According to the invention there is provided a method and means by whicha middle boiling liquid fraction is separated from a mixture containinghigher and lower boiling point fractions. The mixture is passed to theupper portion of a two section fractional distillation unit wherein thetwo sections are separated by a vapor-impermeable plate. Liquid in thebottom portion of the upper section is reboiled and liquid in the lowerportion of the bottom section is reboiled. The middle boiling fractionis removed as a vapor beneath the vapor impermeable plate and there isprovided a means to pass liquid at bubble point from the bottom portionof the top section to the top portion of the bottom section. Accordingto one embodiment of the invention, the means for passing the liquidfrom the top section to the bottom section comprises a downcomer pipewhich extends through the vapor impermeable plate and extends into adoughnut-type tray wherein the liquid level in the doughnut tray isabove the lowest portion of the downcomer pipe. In another embodiment, ahydrocarbon-containing mixture is separated.

The invention will now be described with reference to the drawing whichshows an embodiment of the invention.

Referring now to the drawing, a liquid hydrocarbon stream such as theefiiuent from an isobutane-ethylene alkylation process is fed by line 1to the upper portion of an upper section 2 of a fractional distillationcolumn. Vaporous overhead, containing principally propane, is removedthrough line 17 while the liquid in process passes across a number ofplates 4 and 5 and through downcomers 6 and 7 to bottom plate 8 whichseparates the upper section 2 from the lower section 3 of the fractionaldistillation unit. Plates 4 and 5 are standard fractional distillationunit components which are adapted to pass vapor upwardly through themand are adapted to allow liquid to overflow to a downcomer which feeds alower portion of the unit. Plate 8 is a solid, vapor impermeable platein the preferred embodiment of the invention. The plate, in other words,will not allow vapor or liquid to pass therethrough. A downcomer pipe 9is provided to allow liquid to pass over the upper end thereof, anddownwardly through plate 8 into the lower section 3 of the unit. Liquidis removed by line 10, passed to side reboiler 11, and the vapor-liquidmixture through line 12 and back into the upper section 2, thusproviding stripping vapor. Thus, downcomer pipe 9 supplies liquid fromthe upper section 2 to the lower section 3 at the mixtures boilingpoint. In lower section 3, fractionation plate 13 has weirs 14 at theouter edge thereof to maintain a liquid level on plate 13 above thelevel of the lowest portion of downcomer pipe 9. Thus, liquid ismaintained in downcomer pipe 9 so that vapor cannot pass upwardlythrough pipe 9. Isobutane is removed as a vaporous product from theupper portion of bottom section 3 through line 15. A reboiler 18 isprovided in the bottom portion of bottom section 3 to supply therequisite heat and provide stripping-vapors to the bottom section 3 ofthe unit. Liquid alkylate is removed through line 19.

While both section 2 and section 3 have been shown with 2 and 3fractionation plates each (4, 5 and 13, 16, 17) of the so-calledsplit-cross fiow or doughnut type, a plurality of plates of similar ordiffering types, or of other vapor-liquid contacting devices can be usedas desired.

By the use of the separator plate 8, it has been discovered that thediameter in the bottom section 3 can be somewhat different from that ofupper section 2, in the particular embodiment herein it can be smallerin diameter. Further, it has also been discovered that the diameter ofupper section 2 is smaller than that which would be required if a singlecolumn without plate 8 had been used.

As an example of the apparatus which would be required without plate 8,a tower having a diameter of 18' would be required for the particularfeed rate, composition and desired separation. With the use of plate 8,the diameter of the top section 2 can be reduced to 15 and the diameterof the bottom section can be reduced from 18' to 13 /2. The top section2 would require ten trays and the bottom section 3 would require fiftytrays. Obviously, the smaller diameter unit is less expensive because itrequires less material and is easier to construct, transport and erect.

An additional advantage is obtained by employing the smaller diameterunit. The additional advantage is that a lower heating cost is requiredto separate the constituents of feed stream 1. The cost of providingstripping vapors for the column is roughly in proportion to the squareof the diameters. Thus, the cost of reboiling the upper section of acolumn without plate 8 would be proportional to 18 and of reboiling theupper portion of the column of the invention would be proportional to 15Thus, the ratio of reboiling cost of the prior art column to the columnof the invention for the upper section is 1.44, and similarly, the costof reboiling the bottom section of a prior art column as compared withthe bottom section of the column of this invention is 18 /135 or 1.78.

Thus, it can be seen that the cost of reboiling the column to separatethe components by the invention is substantially reduced from that ofthe prior art.

The column also has advantages over the use of two independentstrippers. The advantages would be that in the invention there is noneed for a bottom head for the first stripper, no need for a top headfor the second stripper, no need for pumps and piping for the bottomflow from the first stripper to the second stripper with additionalutility lines and insulation with the accompanying installation andlabor costs in erecting two columns rather than one. Further, thefoundation and site space for the second column is eliminated by theinvention.

As an example of the use of this method and apparatus, in the separationof the hydrocarbon eflluent stream from a reaction step wherein ethyleneand isobutane are alkylated by a hydrogen chloride-promoted aluminumchloride complex catalyst, a two section fractionation column isemployed ot process approximately 4954 barrels per hour of feed stream.This columns upper section is 15 feet in diameter and contains 10fractionating trays while the lower section is 13.5 feet in diameter andcontains 50 fractionating trays.

The columns feed stream is comprised of 182 barrels per hour of propaneand lighter, 3971 barrels per hour of butanes, predominately isobutane,and 801 barrels per hour of pentane and heavier alkylate, predominatelyisohexanes. Operating at p.s.i.g., overhead pressure, the

overhead vapor stream at 166 F. is removed to further separation stepsas desired at a rate of 1799 barrels per hour and contains isobutaneplus essentially all of the above-mentioned propane and lightercomponents. The sidedraw vapor stream at approximately F. and 163p.s.i.g. is removed at a rate of 2561 barrels per hour, comprisingpredominately isobutane, and is recycled directly to the reaction step.The bottoms liquid product at approximately 320 F. and 173 p.s.i.g. isremoved at a rate of 594 barrels per hour, comprising predominatelyisohexanes alkylate product.

While the invention has been particularly described with reference tothe separation of isobutane from propane and a heavier alkylate, it isobvious that the invention is also applicable to separating othervolatile materials.

Further, the invention can be used for any feed mixture to separate andrecover a low boiling, high boiling and intermediate boiling component.

Reasonable variation and modification are possible within the scope ofthe foregoing disclosure, drawing and the appended claims to theinvention, the essence of which is that there has been provided aprocess and apparatus for separating a middle boiling liquid from amixture of liquids wherein the mixture is fed to the top portion of atwo section fractional distillation unit which has a vapor impermeableplate separating the top section from the bottom section, and a means topass liquid at bubble point from the bottom portion of the top sectionto the top portion of the bottom section, and means for removingvaporous product from the top portion of the bottom section of thefractional distillation unit.

I claim:

1. A separation system for the recovery of a middle boiling fractionfrom a feed mixture containing said middle boiling fraction and higherand lower boiling fractrons, comprising (a) a single fractionaldistillation column containing (1) a top section, and (2) a bottomsection, said bottom section having a smaller diameter than said topsection;

(b) means for passing said feed mixture into said top section;

(0) a solid vapor impermeable plate separating said top section fromsaid bottom section and having as the only fluid passageway through saidplate at least one downcomer;

(d) a tray positioned as the top tray in the top portion of said bottomsection and spaced below said plate to form a vapor space therebetween,said tray having weir means for maintaining a liquid level thereon, saidat least one downcomer extending downwardly into the liquid retained onsaid tray so that said at least one downcomer passes only liquid fromthe bottom portion of said top section to the top portion of said bottomsection while vapor is prevented from passing upwardly from said bottomsection to said top section;

(e) means for removing from said fractional distillation column and fromthe system a primary overhead product stream comprising said lowerboiling fraction by removing an overhead vaporous fraction from the topportion of said top section;

(f) means for removing from said fractional distillation column and fromthe system an intermediate vaporous product stream comprising saidmiddle boiling fraction by removing a vaporous fraction from the vaporspace above said tray in the top portion of said bottom section;

(g) means for removing from said fractional distillation column and fromthe system a bottom product stream comprising said higher boilingfraction by removing liquid from the bottom portion of said bottomsection;

(h) reboiling means for heating liquid in the bottom portion of said topsection to provide stripping vapor for said top section; and

(i) reboiling means for heating liquid in the bottom portion of saidbottom section to provide stripping vapor for said bottom section.

2. Apparatus in accordance with claim 1 wherein said tray is a doughnuttray wherein the liquid level on said tray is above the lowest portionof said downcomer.

3. Apparatus in accordance with claim 1 wherein said at least onedowncomer is a downcomer pipe.

4. Apparatus in accordance with claim 1 wherein said bottom sectioncomprises a bottom portion of smaller diameter than said top section,and a diverging portion connecting the top of said bottom portion to thebottom of said top section, and wherein said means for removing anintermediate vaporous product stream is connected to said divergingportion.

5. A process for recovering the middle boiling fraction from a feedstream containing said fraction and higher and lower boiling fractions,comprising passing said feed stream to the upper portion of a singlefractional distillation zone containing an upper zone and a lower zone,said lower zone having a smaller diameter than said upper zone, saidupper zone and lower zone being separated by a solid, vapor impermeableplate having as the only fluid passageway through said plate at leastone downcomer, the lower portion of said at least one downcomerextending into the liquid retained on a tray positioned as the top trayin the upper portion of said lower zone, passing only liquid throughsaid at least one downcomer from said upper zone to said lower zonewhile maintaining the level of the liquid on said tray above said lowerportion of said at least one downcomer to prevent the flow of vapor fromsaid lower zone to said upper zone, reboiling liquid in said upper zoneto provide stripping vapor for said upper zone, removing a vaporousoverhead product stream from said fractional distillation zone and fromthe process by withdrawing a vaporous stream from the upper portion ofsaid upper zone, heating liquid in said lower zone to provide strippingvapor for said lower zone, removing a bottom product stream from saidfractional distillation zone and from the process by withdrawing liquidfrom the lower portion of said lower zone, and removing an intermediatevaporous product stream containing said middle boiling fraction fromsaid fractional distillation zone and from the process by withdrawing avaporous stream from the vapor space above said tray in the upperportion of said lower zone.

6. A process according to claim 5 wherein said feed stream comprisespropane, isobutane, and a C alkylate, said middle boiling fractioncomprising an isobutane fraction.

7. A process in accordance with claim 6 wherein said lower zonecomprises a lower portion of smaller diameter than the diameter of saidupper zone, and a diverging portion connecting the upper end of saidlower portion and the lower end of said upper zone, said intermediatevaporous product stream being withdrawn from said diverging portion.

8. A process in accordance with claim 7 wherein each of said upper zoneand said lower zone contains at least one liquid holding tray providedwith overflow weirs.

References Cited UNITED STATES PATENTS 2,311,180 2/1943 Bogart et al 2032,366,361 1/1945 Semon et al 20378 X 2,388,919 11/1945 Iverson 203-73 X2,410,498 11/1946 Hepp 260-683.45 2,530,143 11/1950 Baker 260683.532,555,939 6/1951 Sherwin 26075 2,642,388 6/1953 Steitz et al 26073 X2,738,964 3/1956 Binder et a1. 202158 X 2,852,582 9/1958 Stallings eta1. 260--683.53 2,855,448 10/1958 Goard 26068357 2,886,492 5/1959 Hansonet al. 20 375 X 3,357,901 12/1967 Otsuka et al. 20378 3,401,092 9/ 1968Matta 203-1 OTHER REFERENCES American Petroleum Refining, 4th Edition(Bell) pp. 278-284 (1959).

NORMAN YUDKOFF, Primary Examiner F. E. DRUMMOND, Assistant Examiner Us.01. X.R.

